1. Field of the Invention
The present invention relates to an improved burglar alarm consisting of a base having peripherally disposed side wall thereon, forming a concave space for receiving a ring-like magnet which has an extended resilient arm fixed at one end, permitting reciprocating vibration on being actuated by external force, and a reed relay located through the central opening of the ring-like magnet, which is effected by the vibrating magnet so to actuate an alarm apparatus for producing warning signals to the security people or the like. The present alarm device is characterized in its sensitive, reliable and simple structure, free of the external sabotage by magnetic force.
2. Prior Art
There are two most prevailing burglar alarms available on the market, the first one adopts the "reed relay", and the second one uses "vibration detecting means". The first type of device consists of a magnet 421A and a reed relay 41A which is provided with separable magnetic contact points 411A. In practical use, the reed relay 41A is mounted on the frame of a window and connected to an alarm device, and the magnet 421A in case 42A is fixed on the movable window, so the distance between the magnet and relay is varied when the window is pushed opened, resulting in the reduction of the magnetic force therebetween. This causes the contact points 411A of the reed relay 41A come into contact (as shown in FIG. 7 and FIG. 8), and a signal is accordingly produced to actuate an alarm.
The disadvantage of this type of device lies in that when the burglar breaks the window instead of pushing it open, this type of alarm device becomes useless in detecting the breaking-in of the burglar. Therefore, another complementary device which can be actuated by vibration must be mounted on the window glass whereby the breaking of the window will be detected. This type of so called "vibration detecting" device falls in three categories, including a mercury type, counter weight type, and sound actuation type. The operation modes of the three above cited devices are described as below.
1. The mercury type device consists of a sealed glass tube 14A in which short a long metal legs 11A, 12A are parallelly disposed, the tube being connected externally to an alarm device and housed in a shelter case 1A as shown from FIG. 1 to FIG. 3. The external stimulation on the tube will make the fluid mercury 13A move back and forth so to electrically connect the two legs, thus making the alarm device operate accordingly.
2. The counter weight type device has an elongated spring plate 23A, one end of which is fixed on the case 21A and the other end of which is free to move up and down. A counter weight 24A is attached to the free end so to enable the free end to vertically vibrate in a large scale and reciprocatingly contact the point 25A as shown in FIG. 4 and FIG. 5, thereby producing "open and close" signals. Element 22A is an adjusting screw which determines the vibration condition of the counter weight.
3. The sound actuation type adopts a sound detecting device 31A such as a piezoelectric buzzer or a microphone, which is fixed on a protected body; and the sound produced by the impact on the protected body by foreign object will stimulate the sound detecting device and the signal is transmitted to a frequency amplifying transformer 32A to produce an actuation signal.
The disadvantages inherent in the above cited three vibration detecting devices are listed as follows.
1. The mercury type device uses fluid mercury which is not able to be controllably adjusted and is apt to be mistakingly actuated.
2. The counter weight type device has its two contact points constantly exposed to the air and is easily oxidized thereby, causing malfunction accordingly.
3. The sound actuation type needs extra equipment to amplify the produced signals, so the price is higher.